Ethanol can be produced through fermentation and distillation of starches found in feedstock crops such as corn, potatoes and sugar cane. Conventional processes for producing ethanol include dry milling and wet milling. A feature of the dry milling process is that the feedstock is processed without first separating it into its component parts. In a conventional dry milling production process, the starch portion of the crop is used to produce the ethanol while the remaining nutrients, including proteins, oils, minerals and vitamins, are typically concentrated into distillers grain, which is a valuable feed for livestock.
In dry milling, the entire starchy grain, commonly corn, is first ground into flour, which is then mixed with water to form a mash, heated to reduce bacteria levels prior to fermentation, and cooled. Enzymes are added to the mash to convert the starches to sugars. It may be advantageous to add enzymes during the mash-forming step in order to decrease the viscosity of the gelatinized starch, which facilitates ease of handling. The mash is transferred to reaction vessels where the starch is converted by addition of saccharifying enzymes to fermentable simple sugars such as sucrose, glucose and/or maltose. During saccharification, the mixture is preferably maintained at a temperature from about 100° F. to 110° F., more preferably about 105° F. Ammonia may be added to control the pH and as a nutrient for the subsequently added yeast.
The mixture of corn residue, sugar and water is next transferred to fermenters where yeast is added, which initiates the conversion of the sugars to ethanol and carbon dioxide gas. During the fermentation process, the mash is typically agitated and cooled in order to activate the yeast, and the carbon dioxide gas released during fermentation can be captured and used for carbonating beverages or for the manufacture of dry ice. After fermentation, the resulting beer is transferred to distillation columns where ethanol is separated from the remaining stillage.
The stillage, which includes both dissolved and suspended solids, is typically sent through a series of centrifuges and evaporators and then to a rotary dryer to reduce moisture. Prior to drying, the separated solids are referred to as the wet cake, while the output of the drying stage is a co-product called distillers dried grains (DDG).
The solubles are typically concentrated by evaporation. Corn condensed distillers solubles (CDS) is a term generally used to refer to evaporated co-products of the dry milling. In a conventional dry milling plant, most of the CDS are added to the dried grains, but some are available as a liquid feed ingredient. The solubles are an excellent source of vitamins and minerals, including phosphorus and potassium. CDS can be dried to about 5 wt. % moisture and marketed, but generally the water content is between about 50-75 wt. %.
The condensed distillers solubles (CDS) can be dried together with the wet cake to produce distillers dried grains with solubles (DDGS), which are recovered in the distillery and contain all the nutrients from the incoming corn except the starch.
Following fermentation and separation of the ethanol, feedstock oils (e.g., corn oil) can be extracted from the stillage. However, because the stillage is processed through the relatively high temperatures associated with fermentation and distillation, the oils typically are thermally degraded prior to extraction. The thermal degradation of the oils reduces their value.
In addition to the problem of thermal degradation, conventional processes for separating feedstock oils from the post-fermentation stillage typically use hexane-based solvents to facilitate the extraction. Hexane extraction plants are expensive to build, run, and maintain, however, due to the explosion hazard potential and the human and environmental safety issues and regulations.
In view of the foregoing, one object of the invention is to increase the efficiency of the dry milling fermentation process as well as the value output of the fermentation residuals, including the distillers grain and extractable oils.
Applicants' inventive liquid stream capture processes for extracting various components in a dry mill ethanol process achieve this and other objects and advantages of the invention. The liquid stream capture processes can be used to extract converted carbohydrates and sugars from a feedstock mash and, subsequently, to extract the feedstock oils and proteins from the residual solids.
According to one aspect of the invention, the corn residue, sugar and water mixture that is obtained by combining enzymes with the mash is rinsed with a dilute solvent stream in a bifurcating step in order to separate the sugars from the feedstock solids. According to a preferred embodiment, the dilute solvent stream comprises 2% ethanol and 98% water. The solute (sugars and water) can be transferred to fermenters.
According to a further aspect of the invention, after the bifurcating step, a concentrated solvent stream is used in conjunction with a mechanical extractor (such as, for example, a centrifuge) to produce a washout of miscella from the feedstock solids (corn residue). Specifically, after the sugars have been separated from the feedstock solids, the concentrated solvent separates the miscella (water, oils and proteins) from the solids. According to a preferred embodiment, the concentrated solvent stream comprises 100% ethanol.
After extraction of the miscella, adsorbed solvent can be removed from the solids, and the solids can be dried. A preferred apparatus for desorbing solvent from the residual solids is a desolventizer/toaster/dryer/cooler (DTDC) apparatus. Preferably, the recovered solvent is dried and recycled to wash out additional miscella.
In a separate step, the miscella can be separated into its constituent parts (water, oils and proteins) using, for example, fractional evaporation or other thermal processing. Retained solvent that is mixed with the miscella can also be separated using fractional evaporation.
Thus, the inventive method can be used to deliver a stream of sugars and water to a dry mill fermentation process and, after the sugars have been separated from the solids, to further extract useful miscella from the solids. Advantageously, the sugar/water stream is substantially free of suspended solids. By delivering a suspended solids free stream of sugars and water to the fermentation process, the subsequent isolation and purification of the product ethanol is greatly simplified. Moreover, by extracting the miscella from pre-fermentation solids, the oils obtained are not thermally degraded prior to their extraction.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.